The present invention relates generally to a zoom lens system of two-group construction in which each lens group consists of a single lens alone, and more particularly to a zoom lens system which is suitable for use as an image pickup lens for still cameras, video cameras or the like, and which comprises two groups, each consisting of one lens, and has an F-number of 4 or less at its wide-angle end.
So far, some proposals have been made of a zoom lens system of two-group construction, each group consisting of one single lens. Referring to an optical system disclosed in JP-A 7-333500 for instance, the power of the second lens group is too strong to reduce aberration variations upon zooming, making it very difficult to increase lens aperture and zoom ratio. JP-A""s 10-3037 and 11-6960 put forward a zoom lens system of two-group construction using a gradient index medium. In consideration of material availability, etc., however, it is preferable to use a homogeneous medium.
In view of such problems with the prior art as mentioned above, it is an object of the present invention to provide a zoom lens system of two-group construction, each consisting of one single lens, which is less susceptible to aberration variations even upon zooming and can achieve large lens aperture and high zoom ratio.
According to one aspect of the invention, this object is achievable by the provision of a zoom lens system of two-group construction, which comprises, in order from an object side thereof, a first lens group having negative refracting power and a second lens group having positive refracting power and in which a focal length thereof is varied by moving both said lenses while an optical axis separation therebetween is varied, wherein:
said first lens group consists of one concave lens having a strong concave surface directed toward an image plane side of said zoom lens system and formed of a homogeneous medium,
said second lens group consists of one convex lens formed of a homogeneous medium,
a stop is located between said first lens group and said second lens group, and
said first lens group moves from said object side toward an image side of said zoom lens system while said second lens group moves from said image side toward said object side during zooming from a wide-angle end to a telephoto end of said zoom lens system.
Preferably in this case, the convex lens in the second lens group should have at least one surface defined by an aspherical surface.
Preferably, the zoom lens system of the invention should satisfy the following ratio relation:
1.0 less than f2/fw less than 4.0xe2x80x83xe2x80x83(1)
where fw is a focal length at which said zoom lens system has the shortest focal length and f2 is a focal length of the second lens group.
Preferably, the stop should move together with the second lens group while the following relation is satisfied:
0.5 less than {(n2xe2x88x921)/R2F}/(1/f2) less than 1.5xe2x80x83xe2x80x83(4)
where n2 is the refractive index of the convex lens in the second lens group, R2F is the radius of curvature of a surface of the convex lens located on the object side, and f2 is the focal length of the second lens group.
According to another aspect of the invention, there is also provided a display device with a built-in two-group zoom lens system having phototaking function, which comprises:
a first lens group consisting of one negative lens and a second lens group consisting of one positive lens,
an electronic image pickup element for receiving an object image formed by only refracting powers of said first lens group and said second lens group, and a processing circuit for electrically processing information on said object image received by said electronic image pickup element, and
an image display element for displaying the information on said object image fed through said processing circuit to an viewer.
Preferably in this case, the two-group zoom lens system should be constructed in such a way that a separation between the first lens group and the second lens group is varied for zomming from a wide-angle end to a telephoto end of the system.
Alternatively, the separation between the first lens group and the second lens group may be narrowed for zooming from the wide-angle end to the telephoto end of the system.
Preferably, a stop should be located between the first lens group and the second lens group.
A filter may be located between the second lens group and the electronic image pickup element.
A cover member may be located in front of the first lens group.
The first lens group may consist of one cemented lens comprising a plurality of cemented lens elements.
The first lens group may consist of one lens element.
The second lens group may consist of one cemented lens comprising a plurality of cemented lens elements.
The second lens group may consist of one lens element.
The image display element may be built up of a liquid crystal display having a flat screen.
Preferably, the first lens group should have at least one aspherical surface configured to make correction for aberrations.
Preferably, the second lens group should have at least one aspherical surface configured to make correction for aberrations.
In the another aspect of the invention, too, the following ratio relation should preferably be satisfied:
1.0 less than f2/fw less than 4.0xe2x80x83xe2x80x83(1)
where fw is a focal length at which said zoom lens system has the shortest focal length and f2 is a focal length of the second lens group.
Preferably, the stop should move together with the second lens group while the following relation is satisfied:
0.5 less than (n2xe2x88x921)f2/R2F less than 1.5xe2x80x83xe2x80x83(4)
where n2 is the refractive index of the convex lens in the second lens group, R2F is the radius of curvature of a surface of the convex lens located on the object side, and f2 is the focal length of the second lens group.
Preferably, the zoom lens system should be compactly housed in the display device by allowing the first lens group and the second lens group to move in such a way that a distance from a surface located nearest to the object side in the first lens group to the electronic image pickup element is reduced to a length between 20 mm and 2 mm.
Preferably, a separation between the first lens group and the second lens group should be variable in such a way as to satisfy the following zoom ratio condition:
1.2 less than fT/fW less than 25.0xe2x80x83xe2x80x83(6)
where fw is a focal length at which said zoom lens system has the shortest focal length and ft is a focal length at which said zoom lens system has the longest focal length.
According to yet another aspect of the invention, there is provided a telephone handset comprising a display device with a built-in two-group zoom lens system having phototaking function, a microphone for entering an operator""s voice therein, a speaker for producing a voice of a person at the other end and a dial button for entering at least a telephone number therein.
The telephone handset may be constructed in the form of a portable telephone set having an antenna for transmitting and/or receiving radio waves.
Why the aforesaid arrangements are used and how they word will now be described.
In the zoom lens system of two-group construction, one lens for each group or two lenses in all, according to the present invention, the first lens group consists of one concave lens having a strong concave surface directed toward the image plane side and formed of a homogeneous medium and the second lens group consists of one convex lens formed of a homogeneous medium with a stop located between them. For zooming from the wide-angle end to the telephoto end of the lens system, the first lens group is allowed to move largely from the object side to the image side of the lens system while the second lens group is allowed to move from the image side to the object side of the lens system. Thus, the convex lens in the second lens group takes a chief part in the image-forming action of the lens system. This arrangement is favorable in view of aberration correction because the height of a ray passing through the second lens group remains substantially unchanged irrespective of zooming settings. For aberration correction, it is favorable to use an aspherical surface in the second lens group because considerable aberrations are produced at the second lens group.
When the upper limit of 4.0 in condition (1) is exceeded, it is required to increase the separation between the first lens group and the second lens group in a wide-angle setting, resulting in an increase in the entire length of the lens system. When the lower limit of 1.0 in condition (1) is not reached, on the other hand, the power of each lens group becomes too strong to make correction for aberrations with only two lenses.
More preferably, condition (1) should be reduced to:
1.5 less than f2/fW less than 2.5xe2x80x83xe2x80x83(2)
Even more preferably, condition (1) should be reduced to:
1.6 less than f2/fW less than 2.1xe2x80x83xe2x80x83(3)
It is also preferable for the power produced at the front surface in the second lens group to satisfy the following condition:
0.5 less than (n2xe2x88x921)f2/R2F less than 1.5xe2x80x83xe2x80x83(4)
Here n2 is the refractive index of the convex lens in the second lens group, and R2F is the radius of curvature of a surface of the convex lens located on the object side.
In the zoom lens system of the invention, the stop is located between the first lens group and the second lens group, and moves together with the second lens group during zooming. Thus, the front surface in the second lens group is located nearest to the stop. When the stop is at a position near to a principal point, the amount of distortion produced is small. When the power produced at the front surface in the second lens group is short of the lower limit of 0.5 in condition (4), however, the distortion becomes worse because the power of the final surface in the second lens group is larger than that of the front surface, and power at a position away from the stop becomes strong accordingly. When the upper limit of 1.5 is exceeded, on the other hand, aberrations produced at this surface become too large for correction at other surface.
More preferably for aberration correction, the power of this surface should satisfy:
0.9 less than (n2xe2x88x921)f2/R2F less than 1.1xe2x80x83xe2x80x83(5)
Preferably in the zoom lens system of the invention, the separation between the first lens group and the second lens group should be variable in such a way as to satisfy the following zoom ratio (fT/fW) condition:
1.2 less than fT/fW less than 25.0xe2x80x83xe2x80x83(6)
Here fw is a focal length at which said zoom lens system has the shortest focal length and ft is a focal length at which said zoom lens system has the longest focal length.
In one preferable embodiment of the invention, this zoom lens system is used with an electronic image pickup element such as a CCD to set up a display device having phototaking function for an electronic camera or the like. According to this embodiment, the zoom lens system can be compactly housed in the display device by allowing the first and second lens groups to move in such a way that the distance from the surface located nearest to the object side in the first lens group to the electronic image pickup element is reduced to the length between 20 mm and 2 mm.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.